From top to bottom, relativistic broad Fe K_α line equivalent width and β vs. black hole mass, accretion rate and hard X-ray luminosity. Filled circles indicate broad Fe K_α line detections at  ≥ 5σ confidence level (where error bars indicate the 90% confidence level intervals). The dashed vertical lines in the bottom plots indicate the L1, L2, and L3 luminosity bins as described in the text (see Sect. 4.2.3).